This invention relates to radiation-imaging apparatus employing xe2x80x9cgamma camerasxe2x80x9d having coordinate computation circuitry and energy summation circuits for computing the location and the energy of xe2x80x9ceventsxe2x80x9d in such systems, and in particular to high count rate gamma camera systems. This computed information is used to provide images based on the energy and location of the events.
Since the invention of the gamma camera by Anger, scientists have been attempting to improve the count rate of the camera and the camera""s resolution at the higher count rates when used in single photon emission (SPE) imaging systems. Anger gamma camera type systems are now also used in high count positron emission tomographic (PET) systems, using multi-head gamma cameras and coincidence circuits.
The count rate of the camera, that is the number of impingements that can be recorded by the camera per unit time, is a function of the dead time of the camera. The dead time of the camera is a time during which the system processes a single event and is not available to process a succeeding event. The term xe2x80x9ceventxe2x80x9d as used herein means the impingement of a scintillator by radiation stimuli that surpasses a given threshold and cause a scintillation and a consequent electrical signal from light sensors, such as photo-multiplier tubes (PMTs), coupled to the scintillator. Related to the dead time, but separately defined therefrom is xe2x80x9cpulse pile-upxe2x80x9d. A pulse pile-up is a second scintillation that occurs within the light collection time of a first scintillation. In the case of pulse pile-up, the computation system treats the two pulses as one; and thereby computes the energy and location of both scintillations together, which results in an erroneous location and energy. In yet the erroneous computation understandably degrades the image and thus the rejection of pulse pile-up events significantly improves the image. However, the blanket rejection of pulse pile-up events by prior art systems lowers the count rate.
Anger type gamma cameras have been used in single photon emission computerized tomographic systems (SPECT) and planar systems for many years. More recently, Anger cameras have been used in PET systems. In both cases, the introduction of relatively high-speed detection electronics and computer systems for image acquisition and processing has made it even more desirable that the count rate of the Anger type camera be increased. In PET systems, the detection of two gamma rays in different gamma camera heads in coincidence is used to enable computation of imaging information. See, for example, a paper presented by G. Muehllehner et al, entitled xe2x80x9cPerformance Parameters of a Positron Imaging Cameraxe2x80x9d, published in the I.E.E.E. Transactions on Nuclear Science, volume NS-23, No. 1, pp 528-537 (February 1976). See also a paper entitled xe2x80x9cPerformance Parameters of the Longitudinal Tomographic Positron Imaging Systemxe2x80x9d by A. M. J. Paans et al, in Nuclear Instruments and Methods, vol. 192, pp 491-500 (February 1982).
Higher count rates of usable output signals are achieved by decreasing dead time. Rejecting pile-up events improves the image but lowers the count rate. Among the systems used in the past for increasing the count rate have been the use of means for reducing the dead time of the cameras. More particularly, in the past, among the ways for reducing the dead time has been truncation of the pulse provided by the PMTs of the scintillation camera. See, for example, U.S. Pat. No. 4,455,616, the contents of which are hereby incorporated by reference.
Also, in the past, gamma camera images have been improved by, among other things, determining the region in the crystal within which a light event occurs, and coupling to the coordinate computing circuitry only photo-detectors immediately adjacent to the light event. Thus, in the past, it has been known to connect PMTs that are immediately adjacent to the light event to coordinate computation circuitry. See, for example, U.S. Pat. No. 4,100,413, the contents of which are hereby included herein by reference.
Another prior art method for allegedly increasing count rate has been utilization of more than one integrator for each scintillation detector channel enabling the system to collect more than one event per detector at a time in either PET or SPECT mode. (See, for example, U.S. Pat. No. 5,586,637). One of the problems with the system of that patent is that no pile-up rejection is used. The patent contends that no pile-up rejection is required. The multiple integrators, however, do not solve pile-up problems that occur within selected clusters. The pile-up events that are not rejected are contaminated, and their use adversely affects the image. The decision of prior art systems is to discard the pile-up events without taking into account the spatial distance between the events causing the pile-up. This discarding of pile-up events significantly decreases the count rate of the cameras.
It is an aspect of some embodiments of the present invention to take into account both the spatial and time separation between events that are presently considered pile-up events. Preferably, this enables utilization of previously discarded events.
In some embodiments of the invention, multiple events that occur within the dead time of the event detector (xcfx841) ns are considered contaminated and are rejected. In some embodiments, events that are monitored by different clusters of PMTs and occur after xcfx841 ns are used. Also, events monitored by the same cluster and separated by at least the integration time of the detector (xcfx842ns ns) are used.
According to alternate embodiments of the invention, events that are spatially separated can be used even if they are almost simultaneous.
According to an aspect of some embodiments of the present invention, a system of dynamic cluster selection is used. More particularly, a quick-Anger computation is performed in order to quickly obtain coarse X-Y coordinates. The X-Y coordinates may be normalized with the energy that is also quickly obtained and used to select a cluster of PMTs adjacent to the event for processing the signals initiated by the event. In some embodiments, the selection of the cluster is accomplished using a look-up table (LUT), wherein the address is the location defined by the coarse X-Y coordinates and the output selects the PMTs of the cluster. In some embodiments, the PMTs immediately adjacent to the light event location are directly selected by switching circuitry. The cluster will contain PMTs that are proximate to the event and the outputs of which will be used in the Anger computations. To accomplish this, one or more array of analog switches may be activated so that only the light sensors such as PMTs contained in the selected cluster are connected to the regular coordinate computation circuitry. One of the advantages of the dynamic cluster selection is the resultant improvement in homogeneity of the image. Optionally, the connection to the regular coordinate computation circuitry includes a pile-up rejector associated with the region monitored by the cluster.
In some embodiments, all multiple events occurring within a time frame of less than xcfx841 ns of each other are rejected. Multiple events, wherein one of the events is within the selected region and one of the events occurs outside of the scope of the selected cluster of PMTs are not considered pile-up events; even though the events occur within less than xcfx842 ns of each other, both events are used. The event outside the selected region does not interfere with the Anger computation of the current event within the selected region. In this way the system""s dead-time is reduced significantly and the count rate is increased meaningfully; since, a plurality of different clusters are used to process the events within a given time frame and counts formerly rejected are now used. The detection and rejection of pile-up events, within the selected regions, is performed using event detectors and pile-up rejectors. In some embodiments, the PMT located over the peak signal is used as the center of the cluster, or optionally, to determine the position and shape of the cluster.
In accordance with an aspect of some embodiments of the present invention, a gamma camera system with improved count rate is provided. An exemplary system includes: a gamma camera detector including a scintillation crystal that scintillates responsive to impingement by radiation. A plurality of light sensors or scintillation detectors, such as PMTs, are provided for converting said scintillations to electrical signals. A coarse coordinate determining circuit operates responsive to the electrical signals to quickly determine the approximate coordinates of the events. A scintillation detector cluster selector is operated responsive to the determined coordinates of the events to select a cluster of PMTs to monitor the detected events; and a pile-up rejector is used for rejecting pile-up events i.e., that are spatially proximate to the determined coordinates and occur within a time frame less than xcfx841 (about 50 nanoseconds) of a preceding event.
Circuitry can be provided for processing events that would, in prior art systems, normally be pile-up events and not be processed, since time-wise the succeeding event occurs close to a preceding event. However, according to the invention, since the succeeding events are spatially removed from said determined coordinates of the preceding events and are thus outside of the limits monitored by the selected cluster of light sensors, the preceding events and the succeeding events both may be used. Even if only some of the events that prior art systems discarded are used, this represents a significant increase in the number of events used. The used xe2x80x9cpile-upxe2x80x9d events previously discarded (or which degraded the image if they were not discarded) provide higher count rate imaging data, which among other things, improves the camera""s resolution, and consequently the image quality.
According to another aspect of some embodiments of the present invention, a gamma camera system with improved count rate is provided. Within the scope of the invention, the gamma camera system may include SPECT systems and PET systems, jointly or separately.
In an embodiment used in PET systems, a high count rate nuclear camera includes a pair of gamma camera heads for detecting radiation striking the scintillators of the heads. A quick coordinate-determining circuit is provided for each head for determining the coordinates of the point of impingement of the radiation on the radiation detection. A PMT cluster selector in each head operates responsive to said determined coordinates for selecting certain PMTs adjacent to the event location as determined by the coordinates. Coincident circuitry is provided for determining the coincidence of events occurring in each of the heads within a given time frame. Circuitry is provided for rejecting xe2x80x9cpile-upxe2x80x9d events that are proximate to the determined coordinates. Spatially separated multiple events previously rejected by prior art pile-up rejectors can be processed in their individual clusters.
There is thus provided, in accordance with an aspect of an embodiment of the invention, a gamma camera system including: a gamma radiation detector comprising light sensors operative to detect locations of events responsive to gamma radiation impinging on the detector; a light sensor selector that selects light sensors adjacent to the location of the events time; and wherein said system uses preceding and succeeding events when the succeeding events are outside the area monitored by the selected cluster. In an aspect of an embodiment of the invention, the gamma camera system includes analog to digital circuitry to wholly or partially digitize said system. Further, in the gamma camera system, the light sensors provide an aspect of the invention, the sensors are PMTs.
There is further provided a gamma camera system including: pile-up rejector (PUR) circuitry for rejecting as pile-up events, succeeding events which occur while preceding events are being processed when said succeeding events are within an area monitored by a selected cluster. Among other things, the gamma camera system uses preceding events and succeeding events when the succeeding events are in the scope of a different selected cluster and are separated sufficiently time-wise so that the events do not contaminate one another. According to an aspect of an embodiment of the inventor, the time-wise separation is approximately 50 ns. The gamma camera system further comprises location circuitry that processes said signals to determine coarse coordinates of the detected events. In an embodiment of an aspect of the invention, the location circuitry includes at least one fast event detector and at least one fast location computer for approximating the coordinates of said detected events. The location circuitry may further include coordinate computation circuitry for determining the location of the events for imaging purposes. In accordance with an aspect of an embodiment of the invention, the light sensor selector includes: a switch array Mux operated responsive to said fast event detector for selecting a plurality of switch arrays to select light sensors to form said selected cluster. The noted gamma camera system includes a delay circuit for delaying said signals from the light sensors to provide delayed signals. In some aspects of embodiments of the invention, the light sensor selector further includes a fast-location computer to compute the coarse location of the detected event and also switch control logic circuitry for selecting the switch arrays to connect the light sensors closest to the detected event for providing the selected cluster to monitor the event. In addition, according to some aspects of an embodiment of the invention, summation circuits are connected to said selected light sensors through said switch array to sum the signals in order to provide total energy signals; and coordinate computation circuitry 15 included for providing coordinates of the detected event responsive to said delayed signals. An aspect of an embodiment of the gamma camera system, includes shaping circuitry for shaping the delayed signals before the transmittal of the delayed signal to the coordinate computation circuitry; and logic circuitry for enabling the image coordinate computation circuitry. The logic circuit may include a threshold enable circuit for enabling the coordinate computation circuitry when the detected signal is greater than a certain threshold. In some aspects of an embodiment of the invention, the shaping circuitry includes integrating circuitry. The integrating circuitry may comprise a plurality of integrators for each PMT. A gamma camera system, according to an aspect of an embodiment of the invention, including at least a pair of gamma camera heads and coincidence circuitry for determining when there is a coincident event at each of said gamma camera heads. In some embodiments, at least one analog to digital converter for converting the detected analog signals to digital signals is provided.
There is thus provided a high count rate method for a gamma camera system including a gamma radiation detector comprising a scintillation crystal and a plurality of light detectors for converting scintillations to signals; the method including detecting events responsive to gamma radiation impinging on said detector; processing said detected events to approximate energy and coordinates of the detected events; monitoring the detected events from locations adjacent to the events; and accepting preceding and succeeding events, when the succeeding events occur within a dead time caused by the preceding event but are outside the area monitored from location adjacent to the events. The high count rate method of claim 24 includes providing signals responsive to said events. The signals may be analog signals or digital signals or analog signals may be converted to digital signals. The method may include accepting preceding events and succeeding events when the succeeding events are in the area different than the area adjacent to the event. In some aspects of an embodiment of the invention, the events are accepted when there is a time-wise separation between the succeeding and preceding events, such that the succeeding events are not contaminated by the preceding events. In some aspects of the embodiment of the invention, the time-wise separation is approximately greater than 50 ns. The high count rate method may include approximating the coordinates of said detected events. The high count rate method may also include determining the location of the events with greater accuracy. The high count rate method of an aspect of an embodiment of the invention includes delaying said signals and fine coordinates of the detected event may be computed responsive to said delayed signals. The delayed signals may be shaped before computing the fine coordinates of the detected event; and total energy signals may be used to normalize the fine coordinates. According to an aspect of an embodiment of the invention, accurate computation of the coordinates is enabled when the energy of the events is greater than a certain threshold. The high count rate method may include shaping said detected signals. The shaping may include integrating. An aspect of an embodiment of the invention includes converting the detected analog signals to digital signals. The method may include providing the number of coordinate computation circuits equal to the number of switch arrays. In a like manner, the high count rate method may include using a number of coordinate computation circuits approximating the number of light detectors. The method may also include using a number of pulse shapers approximating the number of light detectors. The method may also include limiting the number of pulse shapers to no more than the number of switch arrays.
In an aspect of an embodiment of the invention, a high count rate gamma camera method uses a gamma camera, and includes: determining the locations of preceding and succeeding events; the preceding and succeeding events that occur within a time interval proportional to the light collection time of the camera but are spatially separated are used. The high count rate method may include partially digitizing the gamma camera or wholly digitizing the gamma camera.